heroism, risk, doom, and osr
The main reason I started to pay attention to "OSR" systems and game culture recently, apart from that it's inspired a lot of really imaginative work (quite a lot of trash too, but anything that inspires good stuff will also inspire trash) and that I'm just bored with rule-heavy games, is the emphasis on how what you do matters - and also how, as a GM, you have to make sure there is a choice.
Swingy, lethal combat means that every time you get into a fight, you may die. As a player, you have to be very sure you want to get into a fight at all, and take every opportunity to get an advantage by exploiting what is around you. When you do live through a fight, you feel that much more alive than if you knew the precise risks beforehand and planned through based on probabilities. The same goes for general environmental threats - traps, dangerous unknown areas, etc.
The other side of this is that, as a GM, you have to make sure that there are ways for PCs to avoid fights or gain an advantage, as well as there always being a risk. Characters dying for no good reason is boring and disempowering. It's not exciting being forced into a fight with tough enemies that you can't avoid, or killed by unexpected unavoidable traps. You don't need to plan every possible mechanism out beforehand, but you should at least have an idea of some believable ways that PCs could survive - or if there's no way for them to survive, some information to let them know they shouldn't go there in the first place.
Since buying Mörk Borg, I've become more interested in the aspect of heroism. Mörk Borg definitely has a lethal system but also, everything is doomed. Every day in MB you roll to see whether the next sign of the apocalypse happens - seven signs means the world ends. It doesn't matter what you do, this is prophecy. The world is going to end. (MB says "burn the book" when that happens, though I guess in practice if you wanted to carry on playing you'd go back in time with new characters. I paid money for the book.)
There's heroism in doing things even in the knowledge that they may kill you no matter how well you prepare, and there's heroism in carrying on doing the right thing even in the knowledge that you are doomed. If you have 1HP and you charge into battle to save a friend, knowing that a single random swing of the dice could mean you're disemboweled, that's heroic. If you get up every day in the knowledge that you're eventually doomed yet still keep moving, fight through the existential crisis that we all face IRL, that's heroic.
I mostly GM these days, but one game that I do play is first edition Warhammer Fantasy Roleplay, and I absolutely value the fact that my meticulously-designed character could just die at any point. (He has already nearly died at least twice, requiring the use of Fate Points to save his life, which you don't get back, and right now I only have one left.) The more effort I put into developing his psyche and background, the more I value his mortality - there's more at risk every time there's another chaotic swordfight in a filthy Old World sewer. Whenever he risks his life it means something. In a tightly min-maxed D&D game with careful game balance this just wouldn't be the case. I don't want Thelmund to win, I want him to live.